This invention relates to a gear machine for use as a pump or motor.
Current gear machines are known to comprise a machine casing closed as it axial ends by respective covers and internally housing two mutually engaging gear wheels. The gear wheels are provided on two rotatable shafts, namely the drive and drive shaft, which are supported by two bushes substantially of "8" shape. Gear machines of the compensation type and of the fixed clearance type are in use. In compensated machines, seal gaskets are installed between the bushes and covers to define a space which is hydraulically connected to that part of the machine through which high-pressure fluid flows. During operation, axial thrusts are created along the clearance walls defined between the gear wheel sides and the bushes and act on the latter, to be compensated by the axial thrusts created between the covers and the space defined by the gaskets. The gaskets are preloaded during machine assembly to compensate for the gap between the gear wheels and bushes. A gear machine of the compensation type therefore allows the gap along the clearance walls to be narrowed allows the axial thrusts which arise along the clearance walls to be balanced, and results in high volumetric efficiency for pumps and high mechanical efficiency for motors. However, the machine has certain drawbacks due to the gasket preloading. In this respect, the preloading tends to urge the bushes towards the gear wheels so that on starting the machine there can be considerable friction between the bushes and gear wheels. For operation as a pump, the machine must be connected to a motor (normally electric) which has to overcome this initial separation friction and must therefore have a high starting torque. Consequently a powerful motor must be connected to the pump. It is apparent that the greater the motor power, the higher its cost. For operation as a motor, the machine must be connected to a pump able to feed fluid at high pressure both to overcome that friction and to overcome the initial load provided by the user device connected to the motor. Likewise, a powerful and therefore more costly pump has to be installed. In machines of the compensation type, the greatest inconvenience occurs therefore on starting, by virtue of the friction existing along the clearance walls.
In fixed clearance machines there is no compensation as no seal gaskets are provided along the spaces between the bushes and covers into which the high pressure fluid flows, so that in these machines there is no narrowing of the play existing along the clearance walls. The machine components are machined to give only a small clearance along the clearance walls. In spite of this, because of the axial thrusts which arise on the bushes during operation, the fluid seeps along these clearance walls. On the one hand, this is an advantage on starting because it results in low friction, but on the other hand it is also a serious drawback in that is considerably reduces machine efficiency. Moreover, because of the aforementioned machining, the machine is more costly and the device (pump or motor) upstream of the machine must be overdimensioned.